JP2006524813A - Thin film evaluation method - Google Patents

Abstract

The present invention provides a new measurement method for evaluating very thin solid films (22) by generating a refractive index grating in a gas or liquid medium in contact with the film (22) by means of a laser. In the first embodiment, the excitation elastic wave (25) in the gas or liquid medium modulates the intensity of the diffractive probe beam, and a signal component is obtained at a frequency lower than the frequency of the elastic mode excited by the solid sample. The amplitude of this low frequency component has a correlation with the amount of energy absorbed by the film (22), that is, the film thickness, so the thickness of the metal film on the underlying dielectric layer can be measured, or the metal film can be detected. A method is provided.

Description

  This application claims the benefit of US Provisional Application No. 60 / 463,259, filed Apr. 16, 2003, the contents of which are hereby incorporated by reference.

  The present invention relates to an optical measurement technique for evaluating characteristics of a sample having a thin film structure, for example.

  For example, a non-contact optical measurement method for evaluating the characteristics of a metal thin film formed on a silicon substrate or a dielectric layer has a great demand for industrial process monitoring and process control. One of the most important parameters for process control is the measurement of the thickness of the metal thin film. Currently, the thickness of metal thin films used in microelectronics is typically in the range of 100-200 mm to a few microns, and the state of the art requires the use of thinner films of 100 mm or less. One application that requires the evaluation of metal films thinner than 100 mm is the fabrication of advanced diffusion barriers for copper interconnects. Another envisaged application is the detection of metal residues on top of the dielectric layer, which remains after a polishing step in the copper interconnect processing process, destabilizing the electrical characteristics of the circuit.

  In one known method, called laser-induced transient grating method or impulse-excited heat scattering method (hereinafter referred to as ISTS), the first excitation laser pulse 3, 3 'is a surface acoustic wave (SAW) as shown in Figure 1 This SAW propagates in the plane direction of the film (see the enlarged portion 8). The second probe laser pulses 6, 6 'are diffracted at the surface of the membrane 1 and the sensor 7 measures the SAW frequency. The SAW frequency depends on the film thickness. ISTS is described, for example, in US Pat. No. 5,633,711 (title; evaluation of material properties by light-induced phonons) and US Pat. No. 5,812,261 (title; method and apparatus for measuring thickness of opaque and transparent films) Are incorporated herein by reference.

  The above technique is used without any trouble for the measurement of the thickness of the metal film in the range of 100 to 10 μm. However, it has been found difficult to apply this method to very thin films (<100 mm). The main problem is that the normal SAW wavelength is several microns, and the film thickness of several tens of millimeters is only about 1/1000 of the SAW wavelength. For this reason, the film has little influence on the SAW propagation, and it is difficult to accurately measure the film thickness based on the SAW frequency.

  The signal waveform produced by ISTS on a solid surface has several components due to different physical processes caused by absorption of excitation light. Usually, the component that has the greatest effect on the signal is due to diffraction of the probe beam from the surface “ripple”. Surface displacement due to surface acoustic waves affects the high frequency component of the signal, and displacement associated with the temperature distribution causes an attenuation component on the slow side.

  Another component of the signal is due to the change in the refractive index of the air above the sample surface. When absorbing the excitation pulse on the sample surface, a part of the generated heat is transferred to the air by thermal diffusion. As a result, the temperature of the air rises spatially and periodically. This instantaneous increase in air temperature further leads to excitation of elastic waves. These elastic waves cause a periodic change in the refractive index of the probe pulse and also affect the diffraction of the probe pulse. Since the speed of sound in air is relatively low, the frequency of elastic waves in air is usually smaller than the SAW frequency of the same wavelength. Because of this low frequency, the effects of waves in the air can be easily distinguished from other components in the signal.

The component of the transient diffraction grating signal due to elastic waves in the air has been attracting attention in the past. For example, Young et al., Entitled “Optical measurement of elastic modulus and thermal diffusion of CN film” (J. Mater. Res., Vol. 10) No. 1 (January 1995), no useful information is extracted from this signal component. Therefore, it is desired to use such previously unused incidental information included in the ISTS signal.
U.S. Pat.No. 5,812,261 U.S. Pat.No. 5,633,711 Young et al., J. Mater. Res., Volume 10, Issue 1, January 1995

  An object of the present invention is to detect a very thin metal film using a transient diffraction grating signal component generated by a refractive index perturbation part of a gas or liquid medium in contact with a sample, and evaluate its thickness.

In one aspect, the invention comprises a method of exciting a membrane to evaluate the membrane, the method comprising:
Irradiating the film with a spatially periodic optical excitation field to produce a thermal diffraction grating;
Creating a spatially periodic refractive index perturbation in a gas or liquid medium in contact with the film by heat transfer from the film to the medium;
Diffracting a probe laser beam at the refractive index perturbation in the medium to form a signal beam;
Detecting the signal beam as a function of time to produce a signal waveform;
Determining at least one characteristic of the membrane based on the signal waveform;
Have

  In an embodiment of the present invention, the film is a metal film. In another embodiment, the film is a metal film having a thickness of less than 100 mm. In another embodiment, the membrane is placed so as to cover an underlying layer that is transparent to excitation radiation. In yet another embodiment, the light absorption coefficient of the underlayer at the excitation frequency is smaller than the absorption coefficient of the material of the film.

  In another embodiment, the gaseous medium in contact with the membrane is air.

  In another embodiment, the refractive index perturbation in the gas and liquid in contact with the sample is caused by elastic waves in the medium.

  In another embodiment, the elastic wave in the medium causes a low frequency modulation of the signal waveform.

  In another embodiment, the determining step is performed based on an analysis of the low frequency component of the signal waveform.

  In another embodiment, the determining step comprises analyzing the signal waveform with experimental correction.

  In yet another embodiment, the determining step comprises analyzing the signal waveform with a theoretical model.

  In another embodiment, the at least one characteristic relates to the thickness of the film.

  In yet another embodiment, the at least one characteristic relates to the presence or absence of the film.

  The present invention has many advantages, which will be apparent from the following description, drawings and claims.

  The invention will be better understood with reference to the following drawings.

  In the new method of the present invention, a metal thin film formed so as to cover a dielectric layer on a silicon wafer is usually detected using a wave signal, and the thickness thereof is measured.

FIG. 2 shows a sample 21 having a very thin translucent metal film 22 formed so as to cover a transparent dielectric layer 23 (for example, SiO ₂ ) layer on a silicon substrate 24. The two short laser pulses 26, 26 'form a spatially periodic light intensity pattern having a period 27, as in the prior art method. In the absence of the metal thin film 22, the absorption of the excitation lights 26 and 26 'occurs only in the Si substrate 24. Usually, the thermal conductivity of the interconnect dielectric is much lower than that of silicon, so little heat is transferred to the air side. Therefore, no elastic wave is generated in the air.

FIG. 3 shows signal waveforms measured on a sample having a 0.55 μm thick SiO ₂ film thermally grown on a silicon wafer. The excitation period is 8.86 μm. This waveform does not include a contribution due to elastic waves generated in the air. This is because the metal film 22 is not provided on the sample.

  When the metal thin film 22 is present on the surface of the sample 21, a part of the energy of the excitation pulses 26 and 26 'is absorbed by the film 22 and transmitted to the air side by thermal diffusion. In FIG. 2, this transmission is indicated by the arrow 25. As a result, instantaneous thermal expansion of air and excitation of elastic waves occur, and the refractive index of air changes. The resulting spatially periodic change in the refractive index of air affects the probe beam 6 as a diffraction grating, resulting in a diffracted signal beam 6 '.

FIG. 4 shows signal waveforms measured under the same conditions as the waveforms shown in FIG. In the sample, a 46-Si TiSiN film is chemically deposited on 0.55 μm SiO ₂ on a Si wafer. Therefore, the measurements in FIGS. 3 and 4 differ only in that the latter has an extremely thin TiSiN film 22. It can be seen that the signal waveform is modulated and a slow vibration 200 occurs. Dividing the 8.86 μm elastic wave defined by the spatial period of the excitation pattern by the period of 25.4 ns of the slow vibration 200 gives a speed of 349 m / s, that is, the speed of sound of air under normal conditions. Accordingly, the slow vibration 200 corresponds to a signal component due to elastic waves in the air generated by heat transfer from the TiSiN film 22 to the air above the film. Due to its low frequency, the contribution of elastic waves in the air to the signal can be easily distinguished from other signal components (eg, SAW components that affect the high frequency vibration 100 of the waveform).

  When the thickness of the metal film is zero, there is no signal component due to elastic waves in the air. Therefore, the amplitude of this signal component increases as the film thickness increases within a certain thickness range. As the membrane becomes thicker, the membrane absorbs more excitation energy and consequently more energy is transferred into the air. This tendency can be observed when the film is almost transparent, that is, depending on the material, as long as the thickness is between 100 and 300 mm. This trend is reversed for thick, opaque films. This is because when the film is thick, the heat traversing in the thickness direction of the film is cooled down to the surface of the film, and the amount of heat transferred to the air is reduced.

  Therefore, when the film thickness is less than 100 mm, there is a correlation between the amplitude of the slow vibration 200 in the signal and the film thickness. As a result, the film thickness can be measured using the amplitude of the slow vibration 200.

  In order to identify the amplitude, the “tail” of the signal waveform is fitted with the following function consisting of a sum of an exponentially damped function, a damped oscillation and a constant offset.

波動の周波数ω、位相θ、および減衰時間τ₂は、一つのTiSiN膜試料のデータに基づいて定められ、定められた値に固定される。他のパラメータ、すなわち、A、τ₁、BおよびCは、波動振幅として得られたBの最適なフィット値を用いて、マルチパラメータフィッティングにおいて定められる。図5には、フィッティングの手法を示すが、線201は測定された信号波形を示し、線201の一部と近接する線202は、(1）式により算定された最適フィッティングを示す。

The wave frequency ω, phase θ, and decay time τ ₂ are determined based on the data of one TiSiN film sample, and are fixed to the determined values. The other parameters, A, τ ₁ , B and C, are determined in multi-parameter fitting using the best fit value of B obtained as the wave amplitude. FIG. 5 shows a fitting method. A line 201 shows a measured signal waveform, and a line 202 close to a part of the line 201 shows an optimum fitting calculated by the equation (1).

  FIG. 6 shows the measured amplitude of the slow vibration component of the signal obtained in one set of TiSiN film samples. The figure also shows the results measured by another known method called low angle incident x-ray reflection (XRR). In FIG. 6, a symbol 60 represents experimentally measured data, and a line 61 connecting the symbols 60 represents an interpolation polynomial curve that is used as a calibration curve in subsequent measurements. A good correlation is obtained between the measurement by the method of the present invention and XRR. The fact that the interpolated curve intersects the x-axis at a point corresponding to about 13 mm instead of zero indicates that the film was exposed to ambient air and partially oxidized during the film formation and measurement period. It is shown that. In general, metal oxides exhibit a sufficiently small absorption coefficient compared to metals, and therefore the method of the present invention is sensitive only to non-oxidized residual portions of the metal film 22.

FIG. 7 shows radial profiles of two types of TiSiN films formed on a 200 mm diameter Si wafer having 0.55 μm thermally grown SiO ₂ . FIG. 6 shows data obtained by measuring the amplitude of the slow vibration 200 in the signal by the above-described method and performing experimental value correction as shown in FIG. To improve the signal-to-noise ratio, the data is averaged over 10 values measured continuously by radial scanning. Although the above measurement example uses experimental value correction, it should be noted that this method can further improve accuracy by using a theoretical model having the following steps. The steps are
(1) Calculation of light absorption in a measurement film formed on a multilayer structure. This can be done by methods according to the prior art.
(2) solving the thermal diffusion problem to determine the temperature rise of the gas or liquid medium in contact with the sample; and
(3) calculating the amplitude of the elastic wave generated in the gas or liquid medium;
Have

  The models and methods used to solve the thermal diffusion and acoustic problems (2), (3) of the liquid in contact with the solid sample are prior art.

  The data shown in FIG. 7 represents an example of the practical application of the method of the present invention, which is the thickness and uniformity of the barrier film of a chemical vapor deposited Cu interconnect (thickness ~ 50 mm). Used for evaluation.

  It should be noted that by using other techniques such as the XRR technique and spectroscopic ellipsometry described above, measurements can be performed on 100 mm and thinner metal films. The advantage of the method of the present invention lies in its high sensitivity, because the component of the transient diffraction grating signal due to elastic waves in air is almost completely due to the presence of the metal film. When it is necessary to detect the presence or absence of a metal film, it is particularly significant to use this method for detecting metal residues after, for example, chemical mechanical polishing (CMP) of copper interconnect structures. Another advantage is that this measurement can be performed using standard commercial ISTS instruments, with extremely thin membrane measurements according to the present invention and thicker membrane measurements according to prior art ISTS technology. This can be done with a single device.

  It should be noted that the mechanism described above for acoustic wave excitation in air is equally applicable to other gas or liquid media in contact with the sample. Measurements on samples immersed in liquid can be used for potential applications such as in situ control of the CMP process. The present invention has many attendant advantages, which will be apparent from the description, drawings, and claims.

  The above expressions and illustrations are only examples and cannot be construed as limiting the scope of the claims.

It is a figure which shows the metal thin film test | inspected using the impulse excitation heat scattering method by a prior art. It is a figure which shows the metal thin film test | inspected using the impulse excitation heat scattering method by this invention. Si has an SiO ₂ layer on the wafer is a diagram showing a signal waveform generated in the sample without surface metal film. It has a SiO ₂ layer on a Si wafer, a diagram showing a signal waveform generated in the sample having a thin film of TiSiN which is formed on the SiO ₂ layer. FIG. 3 is a diagram showing a signal waveform represented by an optimal fitting curve according to Equation 1. It is a chart which shows the amplitude of a wave to metal film thickness. It is a figure which shows the example of the radial direction profile of the TiSiN film thickness measured by the method of this invention.

Claims (13)

A method for evaluating a membrane comprising:
Irradiating the film with a spatially periodic optical excitation field to produce a thermal diffraction grating;
Creating a spatially periodic refractive index perturbation in a gas or liquid medium in contact with the film by heat transfer from the film to the medium;
Diffracting a probe laser beam at the refractive index perturbation in the medium to form a signal beam;
Detecting the signal beam as a function of time to produce a signal waveform;
Determining at least one characteristic of the membrane based on the signal waveform;
Having a method.   2. The method of claim 1, wherein the film comprises a metal film.   3. The method of claim 2, wherein the film is a metal film having a thickness of less than 100 angstroms.   2. The method according to claim 1, wherein the film is placed on an underlayer transparent to excitation radiation.   5. The method according to claim 4, wherein the film is placed on the base layer, and the base layer has a smaller absorption coefficient than the material of the film at an excitation frequency.   2. The method of claim 1, wherein the medium in contact with the membrane is air.   2. The method of claim 1, wherein the refractive index perturbation portion of the medium is associated with an elastic wave.   8. The method of claim 7, wherein the elastic wave in the medium causes a low frequency modulation of the signal waveform.   10. The method of claim 9, wherein the determining step is performed based on an analysis of the low frequency modulation of the signal waveform.   The method of claim 1, wherein the determining step comprises analyzing the signal waveform by experimental correction. The step of determining comprises analyzing the signal waveform with a theoretical model having a calculation of light absorption by the film;
Analysis of thermal diffusion causing a temperature rise in the gas or liquid medium in contact with the membrane;
An analysis of the excitation of the elastic waves caused by the temperature rise;
Analysis of the probe beam diffracted by the refractive index perturbation caused by the temperature rise and elastic waves in the medium;
The method of claim 1, comprising:   The method of claim 1, wherein the at least one characteristic relates to a thickness of the film.   The method of claim 1, wherein the at least one characteristic relates to the presence or absence of the film.

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